It is becoming increasingly apparent that many vertebrates including man carry information in their genomes for complete RNA tumor virus expression, including oncogenic potential. This project is concerned with basic studies on the demonstration, isolation and characterization of such covert as well as overt type C RNA viruses from diverse avian and mammalian species including man. A large portion of our present efforts are directed towards the development and use of sensitive in vitro systems for studying the prevalence and natural behavior of such viruses. Diverse approaches are being intensively applied to animal and human tumor systems in efforts to 'turn on' or rescue overt expressions of viral genomes.